Reasons For Introducing Access Methods
Without access methods, a programmer must write a special program for an I/O channel, a processor dedicated to control peripheral storage device access and data transfer to and from main memory. These channel programs are composed of special instructions, called channel command words (CCWs). Programming those is a complex task requiring detailed knowledge of the hardware characteristics. Channel programs are initiated by a STARTIO macro issued by the operating system. This is usually front ended by the Execute Channel Program (EXCP) macro for application programmer convenience. This macro issues an SVC (supervisor call instruction) that directs the operating system to issue the STARTIO on the application's behalf.
Access methods provide:
- Ease of programming - programmer would no longer deal with a specific device procedures, including error detection and recovery tactics in each and every program. A program designed to process a sequence of 80-character records would work no matter where the data are stored.
- Ease of hardware replacement - programmer would no longer alter a program when data should be migrated to newer model of storage device, provided it supports the same access methods.
- Ease shared data set access - an access method is a trusted program, that allows multiple programs to access the same file, while ensuring the basic data integrity and system security.
Read more about this topic: Access Method
Famous quotes containing the words reasons for, reasons, introducing, access and/or methods:
“Here we also see: what this divinity lacks is not only a sense of shame—and there are also other reasons for conjecturing that in several respects all of the gods could learn from us humans. We humans are—more humane.”
—Friedrich Nietzsche (1844–1900)
“... to a poet, the human community is like the community of birds to a bird, singing to each other. Love is one of the reasons we are singing to one another, love of language itself, love of sound, love of singing itself, and love of the other birds.”
—Sharon Olds (b. 1942)
“The man who thrusts his manners upon me does as if he were to insist on introducing me to his cabinet of curiosities, when I wished to see himself.”
—Henry David Thoreau (1817–1862)
“Make thick my blood,
Stop up th’ access and passage to remorse,
That no compunctious visitings of nature
Shake my fell purpose.”
—William Shakespeare (1564–1616)
“The ancient bitter opposition to improved methods [of production] on the ancient theory that it more than temporarily deprives men of employment ... has no place in the gospel of American progress.”
—Herbert Hoover (1874–1964)